The use of sandwich sheets having a thermoplastic material layer between two thin metallic cover layers is advantageous for the realization of lightweight structural concepts in the automotive sector, because with the use of such sheets the weight saving potentials in automobile construction can be further increased. Sandwich sheets can provide various advantageous, often mutually exclusive characteristics, which yield new weight saving potentials. For example, owing to the plastics layer, sandwich sheets have a considerably lower weight than solid sheets, and simultaneously provide high strength values. Furthermore, sandwich sheets have a sound-damping action and offer high rigidity.
A problem with the use of sandwich sheets, however, is that they have an electrically insulating plastics layer that gives rise to problems during welding processes with regard to the formation of a flawless welded connection. As a result, the integration of sandwich sheets into existing metal constructions has proven to be a problem. Owing to the lack of suitability of sandwich sheets for welding, for example for resistance welding or resistance spot welding, to other metallic components, sandwich sheets are therefore commonly adhesively bonded or mechanically joined to one another.
Nevertheless, to permit welding of sandwich sheets, German Patent Application No. DE 10 2011 109 708 A1 discloses a method for the joining of a sandwich sheet to a further metallic component. According to that method, the intermediate layer is melted in the connecting region and is displaced out of the connecting region, such that a welded connection can subsequently be produced by production of electrical contact between the component and the cover layers of the sandwich sheet. It is proposed that the heating of the joining regions be performed by temperature-controlled electrodes or pressing elements. The welding electrodes or pressing elements are, for this purpose, equipped for example with heating elements that are actuated separately by way of a regulator. The construction of the welding electrodes is thus relatively complex, because modifications to the welding electrode body and to the welding electrode cap are necessary. Furthermore, the rate of heating of the thermoplastic material layer needs to be increased in order to achieve the shortest possible cycle times.
U.S. Pat. No. 4,650,951 describes a method in which two sandwich sheets are connected by way of a remotely situated bypass circuit, such that a current can flow via said bypass circuit for heating purposes, and the sheets can subsequently be welded. This, however, leads to problems in the case of very lightweight sheets because, for example, the thin cover layers of the sandwich sheets can delaminate. Therefore, a method for the resistance welding of two composite sheets is proposed, which method uses two welding electrodes which are heated by heating elements arranged around the welding electrodes and which thus, before the welding process itself begins, warm up and displace the plastics layer situated between the cover layers. This, too, results in a modification of the welding electrodes.
German Patent No. DE 548 002 C discloses a method in which a clamp device situated remote from the welding electrode is used to bypass insulating lacquer, paper, or scale layers of the sheets to be welded and in order to burn off the insulating layers, such that welding is made possible. This, however, can lead to damage and delamination of the sandwich sheets. Furthermore, the welding result may be dependent on positioning of the clamp device, which leads to non-uniform welding processes.
Accordingly, there is a need for methods and devices for performing resistance welding sandwich sheets simply, reliably, and with short cycle times.